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1.
For the last two decades, Iceland and other oceanic plateaux have been considered as potential analogues for the formation of the early Earth's continental crust. This study examines the compositions of silicic rocks from modern oceanic plateaux, revealing their differences to Archaean continental rock types (trondhjemite–tonalite–granodiorite or TTG) and thereby emphasising the contrasted mechanisms and/or sources for their respective origins. In most oceanic plateaux, felsic magmas are thought to be formed by fractional crystallization of basalts. In Iceland, the interaction between mantle plume and the Mid‐Atlantic ridge results in an abnormally high geothermal gradient and melting of the hydrated metabasaltic crust. However, despite the current `Archaean‐like' high geothermal gradients, melting takes place at a shallow depth and is unable to reproduce the TTG trace element signature. Consequently, oceanic plateaux are not suitable environments for the genesis of the Archaean continental crust. However, their subduction could account for the episodic crustal growth which has occurred throughout the Earth's history. 相似文献
2.
The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.
The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al2O3 (>>16%), low TiO2 (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite. 相似文献
3.
《International Geology Review》2012,54(2):206-223
We report elemental and Nd–Sr isotopic data for three types of Ordovician volcanic and gabbroic rocks from the Sharburti Mountains in the West Junggar (Xinjiang), Northwest China. Gabbros and Type I lavas occur in the Early Ordovician Hongguleleng ophiolite whereas Type II and III lavas are parts of the Middle Ordovician Bulukeqi Group. Gabbros and Type I lavas are tholeiites with a depleted light rare earth element (LREE) and mid-oceanic ridge basalt (MORB)-like signature with a crystallization sequence of plagioclase–clinopyroxene, suggesting formation at a mid-oceanic ridge. Type II lavas are Nb-enriched basalts (NEBs, Nb = 14–15 ppm), which have E-MORB-like REE patterns and Nb/Yb and Th/Yb ratios. They come from mantle metasomatized by slab melts. Type III lavas are further divided into two sub-types: (1) Type IIIa is tholeiitic to calc-alkaline basalts and andesites, with REE patterns that are flat or slightly LREE enriched, and with a negative Nb anomaly and Th/Yb enrichment, indicating that they were generated above a subduction zone; (2) Type IIIb is calc-alkaline basalts and andesites, which are strongly enriched in LREE with a marked negative Nb anomaly and Th/Yb enrichment, suggesting generation in a normal island-arc setting. The initial 87Sr/86Sr ratios of Type III lavas range from 0.70443 to 0.70532 and ?Ndt ranges from +1.5 to +4.5, suggesting that these melts were derived from mantle wedge significantly modified by subducted material (enriched mantle I (EMI)) above a subduction zone. Contemporary tholeiitic to calc-alkaline basalt–andesite and NEB association suggest that the NEBs erupted during development of the tholeiitic to calc-alkaline arc. We propose a model of intra-oceanic subduction influenced by ridge subduction for the Ordovician tectono-magmatic evolution of the northern West Junggar. 相似文献
4.
The thickness and geothermal gradient of Archean continental crust are critical factors for understanding the geodynamic processes in Earth's early continental crust. Archean tonalite-trondhjemite-granodiorite (TTG) gneisses provide one of the potential indicators of paleo-crustal thickness and geothermal gradient because crust-derived TTG melts are generally thought to originate from partial melting of mafic rocks at the crustal root. In the Western Shandong Province (WSP) of the North China Craton (NCC), two episodes of Neoarchean TTG magmatism are recognized at ~2.70 Ga and ~2.55 Ga which were sourced from partial melting of juvenile crustal components. The ~2.70 Ga TTG gneisses show highly fractionated rare earth element (REE) patterns (average (La/Yb)N = 39), whereas the ~2.55 Ga TTG gneisses have relatively less fractionated REE patterns (average (La/Yb)N = 18). Petrogenetic evaluation suggest that the magmatic precursors of the TTG gneisses of both episodes originated from partial melting of juvenile crustal materials at different crustal depths with residual mineral phases of Grt, Cpx, Amp, Pl and Ilm. Together with the garnet proportion in the residue, the P–T pseudosections of equilibrium mineral assemblages, and the temperature calculated from Titanium-in-zircon thermometer, we estimate the Neoarchean crustal thicknesses as 44–51 km with geothermal gradients of 17 to 20 °C/km for the ~2.70 Ga TTG gneisses whereas the ~2.55 Ga TTG gneisses show lesser crustal thicknesses of 35–43 km with geothermal gradients of 19 to 26 °C/km, with an approximately 10 km difference in crustal thickness. Our estimates on the thicknesses and geothermal gradients of the Neoarchean crust are similar to those (~41 km, ~20 °C/km) of the modern average continental crust, indicating that a modern-style plate tectonic regime may have played an important role in the formation and evolution of the Neoarchean continental crust in the NCC. 相似文献
5.
Petrogenesis of Archaean Trondhjemites, Tonalites, and Granodiorites from Eastern Finland: Major and Trace Element Geochemistry 总被引：17，自引：4，他引：17
The grey gneisses of eastern Finland form the basement on whichthe Archaean greenstone belts were developed. They are composedof orthogneisses emplaced during two distinct magmatic episodes:2.86 Ga (Kivij?rvi gneisses) and 2.65 (Naavala gneisses). Theirmodal and chemical compositions are those of trondhjemites,tonalites and granodiorites (TTG). Both suites show low-K2Ocalc-alkaline differentiation trends (trondhjemitic). The aim of this study is to qualify and quantify the successionof different mechanisms by which the TTG series evolved. Theyoungest process was studied first, and the arguments then appliedin order to go back in time to the older ones. For each one,quantification was arrived at with the major elements, and theseresults provided a basis for calculation with the rare earthelements (REE). Finally the whole model was tested with othertrace elements. The petrogenetic model may be summarized as follows: meltingof the upper mantle to form a tholeiitic crust; melting of thesetholeiites transformed into garnet-bearing amphibolites to yieldthe parental magma of the TTG. The residue of the melt consistedof hornblende, plagioclase, clinopyroxene, and garnet with minoramounts of ilmenite and magnetite (10 < F < 30); and fractionalcrystallization of hornblende, plagioclase, and ilmenite withoccasional allanite and/or zircon in small amounts ((1-F) <40). No matter when they were emplaced during the Archaean, all theTTG of this part of the Baltic Shield arose from similar parentalmagmas. The petrogenetic study has shown that garnet and hornblendewere necessarily residual phases during the melting of the Archaeantholeiites. This constraint is very important, as it impliesthat the Archaean geothermal gradients occurring in subduction-zoneswere much higher than in modern times, thus allowing the partialmelting of the subducted oceanic crust. 相似文献
6.
Origin of the 3500-3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith 总被引：1，自引：0，他引：1
M.J. Bickle L.F. Bettenay H.J. Chapman D.I. Groves N.J. McNaughton I.H. Campbell J.R. de Laeter 《Precambrian Research》1993,60(1-4)
Calc-alkaline plutonic rocks, intruded at 3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm----Nd isotopic systematics of the 3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and Sm---Nd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean. 相似文献
7.
Late Triassic granitoids of the eastern margin of the Tibetan Plateau: Geochronology, petrogenesis and implications for tectonic evolution 总被引：7，自引：0，他引：7
L. Xiao H.F. Zhang J.D. Clemens Q.W. Wang Z.Z. Kan K.M. Wang P.Z. Ni X.M. Liu 《Lithos》2007,96(3-4):436-452
Late Triassic granitoids in the Songpan-Garzê Fold Belt (SGFB), on the eastern margin of the Tibetan Plateau, formed at 230 to 220 Ma and can be divided into two groups. Group 1 are high-K calc-alkaline rocks with adakitic affinities (K-adakites), with Sr > 400 ppm, Y < 11 ppm, strongly fractionated REE patterns ((La/Yb)N = 32–105) and high K2O/Na2O (≈ 1). Group 2 are ordinary high-K calc-alkaline I-types with lower Sr (< 400 ppm), higher Y (> 18 ppm) and weakly fractionated REE patterns ((La/Yb)N < 20). Rocks of both groups have similar negative Eu anomalies (Eu/Eu = 0.50 to 0.94) and initial 87Sr/86Sr (0.70528 to 0.71086), but group 1 rocks have higher εNd(t) (− 1.01 to − 4.84) than group 2 (− 3.11 to − 6.71). Calculated initial Pb isotope ratios for both groups are: 206Pb/204Pb = 18.343 to 18.627, 207Pb/204Pb = 15.610 to 15.705 and 208Pb/204Pb = 38.269 to 3759. Group 1 magmas were derived through partial melting of thickened and then delaminated TTG-type, eclogitic lower crust, with some contribution from juvenile enriched mantle melts. Group 2 magmas were generated by partial melting of shallower lower crustal rocks. The inferred magma sources of both groups suggest that the basement of the SGFB was similar to the exposed Kangding Complex, and that the SGFB was formed in a similar manner to the South China basement. Here, passive margin crust was greatly thickened and then delaminated, all within a very short time interval ( 20 Myr). Such post-collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas, especially where there is no spatial and temporal association with subduction. 相似文献
8.
Quaternary lavas of the normal island-arc basalt—andesite—dacite association in the islands of Java and Bali range from those belonging to tholeiitic series over Benioff-zone depths of ～ 150 km to high-K calc-alkaline series over Benioff-zone depths of 250 km. More abundant and diverse calc-alkaline lavas are found over intermediate Benioff-zone depths. On average, basaltic lavas become slightly more alkaline (largely due to increased K contents) with increasing depth to the Benioff zone. Levels of incompatible minor and trace elements (K, Rb, Cs, Ba, Nb, U, Th, light REE) show a corresponding increase of almost an order of magnitude.Low average Mg-numbers (～ 0.52) and Ni and Cr abundances (15–25 and 35–60 ppm, respectively) of basaltic lavas suggest that few lavas representing primary mantle-derived magma compositions are present. Calculated primary basaltic magma compositions for most tholeiitic and calc-alkaline volcanic centres are olivine tholeiites with 15–30% ol. The single high-K calc-alkaline centre considered yielded transitional alkali olivine basalt—basanite primary magma compositions. These calculated magma compositions suggest that the percentage of mantle melting decreases with increasing depth to the Benioff zone (from >25 to <10%), while the corresponding depth of magma separation increases from ～ 30 to 60 km.Calculation of REE patterns for basaltic magmas on the basis of peridotitic mantle sources with spinel lherzolite, amphibole lherzolite or garnet lherzolite mineralogy, and model REE levels of twice chondritic abundances, indicates that change in the conditions of magma genesis alone cannot explain the observed change in light-REE abundances of basaltic lavas with increasing depth to the Benioff zone. Complementary calculations of the REE levels of mantle sources required to yield the average tholeiitic, calc-alkaline and high-K calc-alkaline basaltic magma indicate that light-REE abundances must increase from 2–3 to 7–8 times chondrites with increasing depth to the Benioff zone. The percentages of mantle melting favoured on REE evidence are lower than those indicated by major-element considerations.The observed variation in incompatible element geochemistry of mantle magma sources is thought to be related directly or indirectly to dehydration and partial-melting processes affecting subducted oceanic crust. The possible nature of this relationship is discussed. 相似文献
9.
Major, rare earth and transition elements, have been determinedon a selection of volcanic rocks from greenstone belts in Rhodesia(2.6 by) and South Africa (3.4 by). In Rhodesia two distinctseries can be recognized: a komatite-tholeiite series whichoccurs early in the greenstone belt evolution and apparentlygrades into a second, calc-alkaline, series at higher structurallevels. Peridotitic komatites reflect higher degrees of partialmelting than any Phanerozoic rocks so far observed and are thereforeused to place limits on the composition of their source. Atlower MgO contents they merge into low K tholeites which havesignificantly higher CeN/YbN and Rb/Sr ratios (at any MgO content)than those observed in modern ocean floor and island are environments.The calc-alkaline series is characterized by andesites whichexhibit a marked heavy REE depletion, but similar light REEand transition metal contents to the more evolved tholeiticrock types. The continuum of compositions from komatiites totholeiites and calc-alkaline andesites suggests that the bulkof greenstone belt volcanics could have been derived by differentialpartial melting, and polybaric fractionation of an essentiallyhomogeneous peridotite source. Late stage dacitic lavas andintrusions probably reflect melting of a more eclogitic sourceregion. The distinctive trace element geochemistry of Archaeanvolcanics, particularly the high Ni and low Yb values of thecalc-alkaline rocks precludes direct comparison with modernisland are associations. Rather the large decrease in liquidustemperatures (500 °C) with increasing structural heightwithin greenstone belts, coupled with the fact that most ofthe volcanics could have been derived from an essentially homogeneoussource, may suggest that greenstone belts developed in a riftingenvironment. It appears unlikely that the tholeiite/calc-alkalineassociation observed in the Archaean may be taken as an indicationof subduction at that time. 相似文献
10.
河北平山英云闪长质片麻岩锆石SHRIMP年代学 总被引：14，自引：0，他引：14
平山板桥沟地区的英云闪长质片麻岩常量元素具有高硅、高铝、高钠和低钾的特点,稀土元素显示强烈分异,具有较高的(La／Yb)。值(34),同时具有明显的Nb,Ta,Ti和P负异常,与太古宙TTG的特征完全一致。阴极发光和背散射研究表明,英云闪长质片麻岩中锆石具有密集震荡环带和较高的Th／U值,表现典型岩浆锆石的特点,但其外侧都有一高亮度(CL)的狭窄变质增生边。SHRIMP测试结果表明TTG的形成年龄为2536．5±9．8Ma,变质年龄为1802±43Ma。推测2500Ga西部块体向东部陆块俯冲形成了TTG,并导致五台陆块与太行山陆块拼合,使华北克拉通化,1．8Ga则为陆内的另一次构造事件。 相似文献
11.
《International Geology Review》2012,54(12):1055-1115
The Isua supracrustal belt (～3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causing a complex mineral paragenesis throughout the area. For example, a high FeO content of metabasites expands the stability field of hornblende to both lower and higher grades. The compositional and mineralogical characteristics above also indicate that the Isua supracrustal belt underwent a single regional metamorphic event, involving minor contact metamorphism and mylonitization; however, weak ocean-floor metamorphism and low-grade regional metamorphism during accretion cannot be ruled out. Metamorphic pressures and temperatures are estimated to be 5–7 kbar from garnet-hornblende-plagioclase-quartz geobarometry and 380–550°C from garnet-biotite geothermometry in Zones B to D. These P-T estimates indicate an intermediate P/T ratio metamorphic facies series. Geological investigations and chronological constraints of the Isua metamorphic belt indicate that the regional metamorphism was related to the subduction of Archean lithosphere, and records a geothermal gradient for the Archean subduction zone that is much higher than geotherms for Phanerozoic subduction zones. The high geothermal gradient may have resulted from the young age of subducted lithosphere and high potential temperature of the mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin, descending oceanic plate, creating many huge granitic (tonalite-trondhjemite-granodiorite [TTG]) batholiths. Slab melting changed the oceanic crust (density = 3.07) to denser garnet-bearing assemblages (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics. 相似文献
12.
Rare Archaean light rare earth element (LREE)-enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites. These Archaean second-stage melts are divided into at least two distinct groups—Whundo-type and Whitney-type. Whundo-type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc-alkaline mafic to intermediate volcanics. Small compositional differences compared to modern boninites, including higher Al2O3 and heavy REE (HREE), probably reflect secular changes in mantle temperatures and a more garnet-rich residual source. Whundo-type rocks are known from 3.12 and 2.8 Ga assemblages and are true Archaean analogues of modern boninites. Whitney-type rocks occur throughout the Archaean, as far back as ca. 3.8 Ga, and are closely associated with ultramafic magmatism including komatiites, in an affiliation unlike that of modern subduction zones. They are characterised by very high Al2O3 and HREE concentrations, and their extremely depleted compositions require a source which at some stage was more garnet-rich than the source for either modern boninites or Whundo-type second-stage melts. Low La/Yb and La/Gd ratios compared to Whundo-type rocks and modern boninites either reflect very weak subduction-related metasomatism of the mantle source or very limited crustal assimilation by a refractory-mantle derived melt. Regardless, the petrogenesis of the Whitney-type rocks appears either directly or indirectly related to plume magmatism. If Whitney-type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern Tongan high-Ca boninites might apply, but with uniquely Archaean source compositions and source enrichment processes. Second-stage melts from Barberton (S. Africa –3.5 Ga) and ca. 3.0 Ga rocks from the central Pilbara (Australia) have features in common with both Whundo- and Whitney-types, but appear more closely related to the Whitney-type. Subduction zone processes essentially the same as those that produce modern boninites have operated since at least ~3.12 Ga, while a uniquely Archaean boninite-forming process, involving more buoyant oceanic plates and very inefficient mantle-source enrichment, may have occurred before then. 相似文献
13.
Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen, eastern China, which recorded processes of extensional collapse of the orogen. The early stage granitoids ( 132 Ma) are foliated hornblende quartz monzonites and porphyritic monzogranites. They are of high-K calc-alkaline series and metaluminous to weakly peraluminous, with high K2O and low MgO contents (Mg# values: 32.0–46.0), they contain high Sr, low Y and heavy rare earth elements (HREE), and have high Sr/Y and (La/Yb)N ratios, without clear negative Eu, Sr and Ti anomalies. The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase, and thus indicate the existence of an over-thickened (> 50 km) crustal root beneath the orogen at 132 Ma. The later stage granitoids ( 128 Ma) are undeformed fine-grained monzogranites, fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry. They belong to a peraluminous and high-K calc-alkaline to shoshonite series, and display a flat HREE pattern and have strong negative Eu, Sr and Ti anomalies, with low Sr/Y and (La/Yb)N ratios. The late stage granitoids are equilibrated with residues rich in anorthite-rich plagioclase, hornblende, ilmenite/titanite and poor in garnet, indicating that the crust of the Dabie orogen became thinner (< 35 km) at 128 Ma. SHRIMP zircon U–Pb ages and changing compositional trends for these two stages of granitoids indicate that the over-thickened crust formed by the Triassic continental subduction/collision under the Dabie orogen remained until the early Cretaceous, and collapsed quickly in a few million years during the early Cretaceous. 相似文献
14.
Cosmas Kongnyuy Shang Muharrem Satir Emmanuel Nkonguin Nsifa Jean-Paul Liégeois Wolfgang Siebel Heiner Taubald 《International Journal of Earth Sciences》2007,96(5):817-841
We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean
high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic
and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity
that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites)
on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one
for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with
Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic
compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting
of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features
from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial
melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite
melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting. 相似文献
15.
Joseph B. Whalen Vicki J. McNicoll Cees R. van Staal C. Johan Lissenberg Frederick J. Longstaffe George A. Jenner Otto van Breeman 《Lithos》2006,89(3-4):377-404
Silurian plutonic suites in the Newfoundland Appalachians include abundant gabbro, monzogabbro and granite to granodiorite and lesser quartz diorite and tonalite. Most are medium- to high-K, but included are some low-K and shoshonitic mafic compositions. Felsic rocks are of both alkaline (A-type or within-plate granite (WPG)) and calc-alkaline volcanic arc granite (VAG) affinity. Mafic rocks include both arc-like (Nb/Th < 3) calc-alkaline and non-arc-like (Nb/Th > 3) transitional calc-alkaline basalt to continental tholeiitic affinity compositions. εNd(T) values range from − 9.6 to + 5.4 and δ18O (VSMOW) values range from + 3.1 to + 13.2‰.
A rapid progression from exclusively arc-type to non-arc-like mafic and then contemporaneous WPG plus VAG magmatism has been documented using precise U–Pb zircon dating. Earlier arc-like plutonism indicates subduction, while asthenosphere-derived mafic magmas support slab break-off, due to subduction of a young, warm back-arc basin. Contemporaneous mafic magmas with arc and non-arc geochemical signatures may reflect tapping of asthenospheric and subcontinental lithospheric mantle (SCLM) sources and/or contamination of asthenosphere-derived magmas by SCLM or crust.
The brevity (< 5 Ma) of the mafic magmatic pulse agrees with the transient nature of magmatism associated with slab break-off. The subsequent ca. 1 to 2 m.y. period of voluminous WPG and VAG plutonism likely reflects mafic magma-driven partial melting of both SCLM and crustal sources, respectively. Continuation of VAG-like magmatism for an additional 2 to 5 m.y. may reflect lower solidus temperatures of crustal materials, enabling anatexis to continue after mantle melting ceased. East to west spatial variation of εNd and (La/Yb)CN in Silurian plutons suggests a transition from shallow melting of juvenile sources proximal to the collision zone to deeper melting of old source materials in the garnet-stability field further inboard.
Previous work has demonstrated that geochemical discriminaton of post-collisional granitoid magmatism (PCGM) is difficult in the absence of other constraints. Our example should contribute to the understanding and identification of PCGM if it can be employed as a ‘fingerprint’ for slab break-off-related PCGM within the Paleozoic geological record. 相似文献
16.
V Cvetkovi&#x; D Prelevi&#x; H Downes M Jovanovi&#x; O Vaselli Z Pcskay 《Lithos》2004,73(3-4):161-186
Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc–alkaline basalts, shoshonites (HKCA–SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed.
The ESPEMAR (62–39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr–Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and “bell-shaped”, characteristic of an OIB source free of subduction component. 87Sr/86Sri and 143Nd/144Ndi isotope ratios (0.7030–0.7047 and 0.5127–0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR).
The HKCA–SHO rocks (30–21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc–alkaline arc-type magmatism. This is coupled with somewhat enriched Sr–Nd isotope signature (87Sr/86Sri=0.7047–0.7064, 143Nd/144Ndi=0.5124–0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA–SHO mantle source with fluids/melts released from subducted sedimentary material.
UP rocks (35–21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000× primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, LaN up to 400). The isotopic ratios approach crustal values (87Sr/86Sri=0.7059–0.7115 and 143Nd/144Ndi=0.5122–0.5126), and that signature is typical for ultrapotassic rocks worldwide.
The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA–SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes.
To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction–termination/collision stage (Paleocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/early Miocene). 相似文献
17.
Geochemical constraints on the origin of Mesozoic alkaline intrusive complexes from the North China Craton and tectonic implications 总被引：28，自引：0，他引：28
Mesozoic alkaline intrusive complexes are widespread in the southern portion of the North China Craton and can provide some important constraints on the evolution of the Mesozoic lithosphere beneath the region. Three selected intrusive complexes (Tongshi, Hongshan, and Longbaoshan) are generally high in alkalis (K2O+Na2O=913 wt.%) and Al2O3 (1421.6 wt.%) and low in CaO and TiO2 (<0.6 wt.%), with high and variable SiO2 contents. Rocks from these complexes are all enriched in LREE and LILE (Cs, Rb, Ba, U, Th), depleted in Nb and Ti, have a highly positive Pb anomaly, and are characterized by lack of a clear Eu anomaly despite trace element abundances and isotopic ratios that vary greatly between complexes. The Tongshi complex has high Cs (2.68.5 ppm) and REE abundances (∑REE=112.6297 ppm, (La/Yb)N=13.130.9) and MORB-like Sr–Nd–Pb isotopic ratios ((87Sr/86Sr)i<0.704; εNd>0; (206Pb/204Pb)i>18). The Hongshan complex has low REE concentrations (∑REE=28.2118.7 ppm, (La/Yb)N=4.614.7) and is moderately enriched as demonstrated by their Sr–Nd isotopic ratios ((87Sr/86Sr)i>0.706; εNd<−7). The Longbaoshan complex is extremely REE enriched (∑REE=211.3392.6 ppm, (La/Yb)N=32.460.9) and has an EM2-like Sr–Nd isotopic character ((87Sr/86Sr)i>0.7078; εNd<−11). We suggest that the Tongshi complex originated from the asthenosphere and the Hongshan complex and the Longbaoshan complex were derived from the partial melting of previously subduction-modified lithospheric mantle, in response to post-collisional lithospheric extension and asthenospheric upwelling. The occurrence of these alkaline intrusive complexes demonstrates that the lithosphere beneath the region must have been considerably thinned at the time of intrusion of these complexes. This study also shed light on the temporal evolution of the Mesozoic lithosphere and the timing of the lithospheric thinning. 相似文献
18.
Within the metamorphic basement of the Coastal Cordillera of central Chile, the Western Series constitutes the high-pressure (HP)/low-temperature (LT) part (accretionary prism) of a fossil-paired metamorphic belt dominated by metagreywackes. In its eastern part, blocks derived from small lenses of garnet amphibolite with a blueschist facies overprint are locally intercalated and associated with serpentinite and garnet mica-schist. Continuously developed local equilibria were evaluated applying various independent geothermobarometric approaches. An overall anticlockwise PT path results. The prograde path evolved along a geothermal gradient of 15 °C/km, passing the high-pressure end of greenschist facies until a transient assemblage developed within albite-epidote amphibolite facies transitional to eclogite facies at peak metamorphic conditions (600–760 °C, 11–16.5 kbar; stage I). This peak assemblage was overprinted during an external fluid infiltration by an epidote blueschist facies assemblage at 350–500 °C, 10–14 kbar (stage II) indicating nearly isobaric cooling. The retrograde equilibration stage was dated with a Rb–Sr mineral isochron at 305.3±3.2 Ma, somewhat younger (296.6±4.7 Ma) in an adjacent garnet mica-schist. Localized retrograde equilibration continued during decompression down to 300 °C, 5 kbar. The retrograde evolution is identical in the garnet amphibolite and the garnet mica-schist.
The counterclockwise PT path contrasts the usual clockwise PT paths derived from rocks of the Western Series. In addition, their ages related to stage II are the oldest recorded within the fossil wedge at the given latitude. Its “exotic” occurrence is interpreted by the path of the earliest and deepest subducted material that was heated in contact with a still hot mantle. Later accreted and dehydrated material caused hydration and cooling of the earliest accreted material and the neighbouring mantle. After this change also related to rheological conditions, effective exhumation of the early subducted material followed at the base of the hydrated mantle wedge within a cooler environment (geothermal gradient around 10–15 °C/km) than during its burial. The exotic blocks thus provide important time markers for the onset of subduction mass circulation in the Coastal Cordillera accretionary prism during the Late Carboniferous. Continuous subduction mass flow lasted for nearly 100 Ma until the Late Triassic. 相似文献
19.
埃达克岩的原义、特征与成因 总被引：24，自引：2，他引：22
论述了埃达克岩的原义与综合特征 ,并针对其与太古宙TTG之间的区别和联系及今后研究埃达克岩的建议提出了自己的见解。埃达克岩 (adakite)的原义是指一类具有镁铁质斑晶的隐晶质火山岩 ,属于岛弧型岩浆钙碱性岩系 ,一般形成于年轻的 (<2 5Ma)、地热高的岛弧环境 ,是俯冲板块和上覆地幔相互作用产生的杂化熔液通过结晶分异形成的。综合总结埃达克岩 (原义 )的地球化学特征如下 :(1)原生标志 ,高Mg# 、低FeO /MgO、高Cr及Ni;(2 )微量元素标志 ,高LILE、高LREE、低HREE、低HFSE以及高分异的REE型式等。对实验岩石学研究资料的总结可知杂化 (hybridized)熔液是由小数量的板块熔液与地幔楔反应经交代作用、同化作用形成的 ,可分异直至出现酸性岩浆 ,这一过程称为“地幔同化及分离结晶作用 (mantle AFC)”。在橄榄岩的同化作用中 ,原有熔液Mg# 迅速上升 ,并在熔液成分加多后 ,向高Mg# 区迅速发展。在近代一些埃达克岩及相关岩石研究中 ,部分学者认为太古宙TTG与新生代板块 (榴辉岩 )重熔的TTD岩系类似。同时 ,亦有学者认为太古宙“绿岩带”中与TTG有关的深成岩系是一类Mg质花岗闪长岩Mg质二长闪长岩 ,成因与Sanukite相似 (太古宙sanuk itoid岩系 ) ,相当于富集橄榄岩重熔形成的岩系。这些研究重新引发了太古宙 相似文献